The invention pertains to a process for the control of an extruder and the application of the process for the production of extruded section bars.
Extrusion is a well-known process which is applicable in many cases for the manufacture of section bars by extruding materials like for e.g. metal, glass or plastics through a die, whereby the die can possess an opening with almost any cross section from circular to complicated patterns and can have one or more orifices.
Referring to FIG. 1, extruder 10, as known in the art consists essentially of a receptacle 12 with a cylindrical bore 13 of any cross section which accomodates the material to be pressed, usually in the form of a cylindrical billet 14, and a ram provided with a press-disc, whereby a 18 die is provided at one end of the cylindrical bore 13 of the receptacle 12.
In the manufacture of extruded section bars 20, the metal to be extruded is loaded into the cylindrical bore of the receptacle and by applying a high axial pressure via the pressure disc is pressed through the die, so that the material takes on a plastic state under the given temperature and can be extruded, as profile or bar 20, through the opening in the die 18.
In the extrusion of crystalline or vitreous material, the cross section of the section bar corresponds to the cross section of the die opening. However, this does not hold for the extrusion of polymers with structure-viscous (decrease in the viscosity with increase of mechanical stress), entropy-elastic (expansion of the section) and visco-elastic (time dependant coupling of viscosity and elasticity) properties.
The plastic deformabilty of the material to be extruded, and with that the amount of material extruded per unit time, depends upon--apart from the composition of the extruded material and the pressure applied--mainly on the process temperature. To attain the highest extruder speed possible in this thermal conversion process, the exit temperature is kept as high as possible. The maximum possible exit temperature lies on the one hand below the melting point of the extruded material and on the other hand is determined by the condition, that the section bar coming out of the die should not be deformed in the hot state. Furthermore, the bar exit temperature has considerable influence on the material properties of extruded section bars and consequently on the product quality (homogenity, mechanical stresses etc.). Consequently, also due to reasons of quality control, there is considerable interest to prescribe and maintain a definite constant section bar exit temperature in the process. Such a process with a predefined exit temperature which is made to be constant is termed as isothermal extrusion.
The balance of the energy components is obtained from the difference between all the energy inputs (mechanical work and heat) and the outgoing energy (plastic shaping, heat conduction). Here the essential energy components for the heat shaping process refers to the part of the extruded material block which changes its plastic dimensions. The resulting temperature of the section bars when leaving the die can be specifically influenced through the pre-heating temperature of the billets and the extrusion speed.
The practical implementation of isothermal extrusion requires complete knowledge and mastery of all process parameters and in particular all thermal process variables, which is the reason why this process contains many problems for which no technologically satisfactory solutions have been found. Such problems are generally attacked by using known control system methods such as simulated or controlled isothermal extrusion.
In simulated extrusion the exit temperature is calculated in advance through a simulation model, whereby the extrusion speed is the relevant process parameter for control purposes. The extrusion process is however a complicated thermo-mechanical system with many parameters which are not easily incorporated in the model, so that the analytical description of the whole extrusion process is incomplete and the description with numerical methods is imprecise. This is the reason why this method is not suitable for control of extrusion.
In the case of controlled extrusion, the establishment and maintenance of the desired extrusion exit temperature considered as the control variable is obtained through a closed loop control which calculates the necessary extrusion speed correction by constant comparison of the desired and actual values of the control variable. A radiation pyrometer 22, as shown in FIG. 1, is usually used for the measurement of the extrusion exit temperature.
The pyrometric temperature measurement is performed by exploiting Planck's radiation loss which however holds only for ideal black bodies. If the total energy of the emitted radiation is known, then the temperature can be calculated from the measurement of the energy in a certain spectral region by using Planck's radiation law, whereby the temperature represents the temperature which the body would have if it were a black body. As most of the objects are not ideally black, the true temperature is higher than the one calculated in this way. In order to calculate the temperature of a real object, the emissivity, that is the radiation capability of the considered body, should be known. The emissivity of an opaque body is defined as the quotient of the energy emitted by the body and the energy emitted by an ideally black body at the same temperature. The emissivity can be physically described by means of a multiplicative emissivity factor (.epsilon.) which appears in Planck's radiation law. An ideal black body has the emissivity degree .epsilon. equal to 1.
The contactless pyrometric temperature measurement leads however, in the case of materials with small and/or wavelength-dependent emissivity (.epsilon.&lt;0.1) and/or variable surface characteristics, as for example material consisting of aluminium or aluminium alloys, often to a wrong temperature measurement. Therefore, controlled extrusion is not implementable for such materials. In the DE-OS 34 04 054 a production line for isothermal extrusion is described in which an open loop control gives always the same extrusion speed curve v(t) equivalent to the equation EQU v(t)=v.sub.1 +(v.sub.0 -v.sub.1)exp(-At)
for a batch of material, such that the extrusion corresponds to isothermal extrusion process inside a batch even without feedback of the measured temperature run. Thereby v.sub.0 and v.sub.1 denote the initial extrusion speed and the extrusion speed in the steady state of the extrusion process respectively and A a parameter which depends on the mechanical properties of the extruded material as for example the tensile limit which must be measured in the beginning of the batch. For the calculation of v.sub.0 and v.sub.1 a strongly simplified model of the extruder given by EQU .nu.(t)=.nu..sub.1 -(.nu..sub.1 -.nu..sub.2)exp(-Bt)
is used whereby .nu.(t) is the time-dependent exit temperature of extruded material, .nu..sub.1 the temperature of the ram in the stationary stage of the extrusion, .nu..sub.2 the temperature of the billet and B a parameter which represents the mechanical properties of the billet.
A disadvantage of the open loop control described in DE-OS 34 04 054 is to be found in the rigid pre-defined structure of the input function which consists of an exponential part and a constant function part. Such a form of a curve is often not suitable to achieve constant exit temperature. Furthermore, changes in the thermal balance of the extruder as for example, changes in the receptacle temperature, the tool temperature or the billet temperature inside a batch are not taken into account in this process. The model defined by means of the relation of .nu.(t) of the extruder consists of a constant term and an exponential term and thereby represents only very roughly the complicated thermal balance of the extruder.